Resistance grid



M. A. DU Bols 2,128,222

RESISTANCE GRID Aug. 30, 1938.

F'iled Dec. 2, 1935 2 Sheets-Sheet l l l f any wup:-

Patented Aug. so, 193s 2,128,222

UNITED STATES PATENT OFFICE RESISTANCE Gam Marvin A. Du Bois, Chicago,Ill., 'assignon by mesne assignments, to The Post GloverElectrkgiompany, Cincinnati, Ohio, a corporation of` o ApplicationDecember 2, 1935, Serial No. 52,531

6 Claims. (Cl. B01-89) The p'resent invention relatesmtp electricalrelight weight, and designed to have a high degree sstance grids andelectrical resistances contanof practically. uniform self-ventilation.ing the same and has for an object the provision A further object is toprovide resistance devices of a novel and relatively inexpensive grideleof the character described wherein the grid element and, as a furtherobject, the provision of r ments of a given gauge of sheet stock and ata an effectively ventilated, long lived and compact predeterminedspacing have a higher continuous electrical resistance f or use invarious installacarrying capacity than heretofore known grids tions. ofsimilar thickness and width and like spacing A further object of theinvention is to provide that have equal individual resistance. 10 anovel method of forming highly eiilcient resist- Another object is asimple and inexpensively l0 ance grids of sheet metal with a minimum ofloss practiced method of cutting grids of the invenof sheet stock inscrap or waste. tion from an elongated steel sheet.

Another object is to provide a stamped sheet These and other objects areattained by the metal resistance grid characterized in that each methodand means herein described and disgrid, between the terminals thereof,is constituted closed in the accompanying drawings, in which: 15 by acontinuous series of angulariy related sec- Fig. 1 is a plan viewshowing in full lines the tions so disposed with relation to each otheras resistance grid in relation to the sheet in which to provide for thecloser disposition of the sec it is cut, and in dotted lines, theposition of the tions thereof nearest the edges of the grid, and die tocut a succeeding grid.

for the wider spacing of certain non-adjacent Fig. 2 is a crosssectional view of a resistance 20 sections providing for the more readydissipation device embodying the grid oi the invention. oi heat from themore closely spaced sections to Fig. 3 is a view taken on line 3-3 ofFig. 2. the edges of the grid, the said relation of these Fig. ,.4 isacross sectional view of a resistance continuous sections furtherpermitting of the device showing a modiiled form'of mounting the 25stamping of successive grids from a ribbon or grids and of providingadditional support there- 25 50 Another object is to provide electricalresistelongated sheet of electrical steel in a fashion for as may berequired in rare instances -of exproductive of a minimum of waste of thesheet cessively close spacing and very thin grids.

stock from which the grids are stamped. Fig. 5 is a fragmental schematicview showing Another object of the invention is to provide a modifiedform of the mounting of the grids of a resistance grid of stamped sheetelectrical steel the invention in multiple banks. 30', that is designedto provide a high continuous car- Fig. 6 is an enlarged view taken online 6--8 rying capacity factor in proportion to the resistof Fig. 5.ance thereof and which in use in a resistance In the art 0f ConstructingreSiStOrS. Practice device will readily combine the desirable factorshas been largely with cast metal grid resistors of very nearly uniformtemperature distribution or the so called ribbon resistors, and althoughthe :3,5`V

throughout the major portion of each grid with use of sheet metal is notunknown, previous ata surprising1y low temperature at the terminalstempts to use it have resulted in products infethus. avoiding thedestructive action of heat on rior t0 the present device and incurringlarge the insulating supports therefor and upon the Waste Of materialThe Present inventien DIO- insulation and other parts of the actuatorsand Vides a new Sheet metal grid which has advan' 40 lead Wiresassociated with the termina1s tages over the cast grid resistors in thatit gives Another object of the invention is to provide a more evendistribution wheat on the grid and sheet metal resistance grids ofrelatively thin 1s C00] on the ends of the grid during use' allow' gaugeand electrical resistance devices compris- .ing the support to remainC001 and thus retard' y ing the carbonization of the mica tubing. Fur-45 11g he sam Wlilten ghe gridsf may ibe ver thermore, the 4connectingterminals will therefore hls; sm wovglategrobugf ngurf remain cool andthe possibility of destroying insuth id g g lation on wires connected tothe terminals is e gr S- reduced. The devices made in accordance withthe present invention also have specific advanance deViCeSCOmpriSinggrids the afOIeStated tages over cast grid resistors in that they arecharacteristics ilhi Ieqilire a minimum -Of SUD- non-breakable and willwithstand high overloads porting meteriel'end Which ere easily endrapidly without warpirig or drooping o f the grids whereas assembled andwhich, in general, provide an many cast-grid resistor grids droop whenhot so ideally eiiicient resistance device of compact form, that theloops of the grids will touch each other. 55

In addition to this the resistance banks of the present invention willrequire a lesser amount of space than required for a cast grid resistorof equal resistance and carrying capacity because of the thin crosssection of the grid which allows a higher ratio ofY airspace to space.taken up by grids. In addition to this the entire structure is lighterin weight for equal carrying capacity and resistance. Furthermore, thepresent device offers a lower thermal coefficient of resistance. Thedevice furthermore has advantages over ribbon type resistors in that noporcelain need be used so that the resistor may be heated and drenchedwith water without danger of damage. Furthermore, the number ofsupporting bolts is less and of those used a portion can be arranged forfloating mounting so as to allow for expansion. The type of terminalnecessary may` be such as can be firmly attached but easily moved whendesired.

The individual grid of the invention indicated generally by thereference numeral i0 is an integral stamping from suitable electricalsheet steel having a pair of end lugs ii with perforations I2 thereinconnected by a dat strip forming a circuitous path and comprising asuccession of oppositely extending open ended loops connected in aseries between said lugs. The shape of the loops is such that a pair ofclosely spaced sections II which are of uniform cross section and sizeand having a narrow space I4 between, are connected together at a commonend Il and each of these sections i3 in turn connects at its remainingend with an angular-ly divergent portion i6. The portions i6 in turnconnect the sections I3 of the next adjacent oppcsitely extending loopsor with the perforate end lugs Il. Thus it will be noted that the narrowopening Il between the sections I3 of each loop communicates with awider open? ing I1 which is bounded by sections i3 on adjacentoppositely extending loops and the intervening portions Il. It willfurther be noted from a further inspection of Fig. 1 that each of thelarger openings Il is occasioned by the cutting out of the loops of apreceding or succeeding grid. Hence, the amount of scrap or waste stockoccasioned by the production of these grids according to the presentinvention is very small. and in practice represents a seventy-ve percent savingof scrap metal over heretofore known methodsof formingstamped sheet metal resistance grids. This factor of saving is deemedimportant because the nature of electrical sheet steel is such as torender the value of its scrap very low as compared with its initial costbecause this type of metal does not have a wide field of utility forother purposes. Furthermore, a greater number of grids can be stampedfrom a given piece of metal than by heretofore known methods.

The method of forming these grids consists in feeding a steel sheet orstrip i8 endwise beneath a punch and die, the sheet Il in Fig. 1 beingindicated in full lines and exclusive of the completed grid shown inposition prior to the next operation of the die, the position of whichis shown in dotted lines. Thus a complete grid is struck from the sheetat each operation of the die and the outline of half of the succeedinggrid is cut from the sheet in the same operation. This affords a veryrapid and inexpensive fabrication of the metal into grids with a minimumof scrap which will be readily apparent from an inspection of Fig. 1.The method of forming the grids may be stated to be constituted by thesteps of successively stamping out the point shaped openings I1 and theirregular openings I9. With each stroke leaving the forward profile ofthe grid with tongues shaped like opening I4 on the sheet and. on thenext succeeding operation of the die, cutting a similar outline whichsevers the said tongues.

The shape of the grid thus formed is, as before indicated, not onlyproductive of a maximum number of grids with a minimum amount of wastefrom a given quantity of sheet material, but it also affords theutilitarian features of a substantially even distribution of the heatover the major area of the grid and a comparatively low temperature atthe terminal lugs thereof. It will be further noted that there is forall practical purposes uniformity of cross section of resistance metalin the circuitously shaped strip between the end lugs. The relativelyclose proximity of the sections il of a given loop is compensated by thefact that around all sides and ends of the said sections Il there is anenlarged Ventilating area communieating with the outside atmosphere andtending to effect relatively uniform distribution and dissipation ofheat. It has been found that in use this uniform heating and efficientventilation precludes warping of the grids when carrying rated currentand that when the current is increased to bring the grids to a whiteheat, there is but a slight amount of warping which is not sufficient tocause even the more closely spaced grids to touch at any point.Furthermore, when the grids are allowed to cool. they return topractically original position. The warping just referred to obtainedonly under extreme conditions and under conditions wherein both of thelugs Il were rigidly fixed. As will be hereinafter described in greaterdetail, it is possible to eliminate observable warping by mounting thelugs of a bank of grids on a fixed through bolt at one end and on afloating through bolt at the other end so that when the grids areexcessively overheated, e. g., to a white heat, the grids are allowed toexpand so that the distance between the fixed and floating bolts wouldincrease slightly but sutilciently to accommodate the elongation due toexpansion.

The grids of the invention adapt themselves to the mounting in banks asillustrated in Fig. 2 or in a connected series of banks as suggested inFig. 5. They are further adapted to mounting in double banks assuggested in Fig. 4 and in the latter embodiment when the grid thicknessis #18 gauge or thinner and the grid spacings are very small, it hasbeen found desirable to use the center bolts and spacers illustrated inFig. 4 in order to preclude warping and possible touching of the rigidlysecured grids under continued overload heating. The device of Fig. 2 mayhave a center bolt and spacers applied thereto if the grids are thin andare closely spaced.

It is well understood in the art that resistors of various types, forexample, such as illustrated herein, are adapted for use upon electriclocomotives such as mine locomotives, in mills. and in variousenvironments. and that in the design and construction of the variousdevices and apparatus employing these various resistors. it frequentlyhappens that the design or lay out has been made with compactness as animportant object and that no particular provision has been made for theexact location of the necessary resistors. For all such conditions. itis particularly desirable and useful to have a fully efficient resistoras small and compact as possible so as to permit the installation of theresistors if need be in restricted remaining. space or spaces. In

the following detailed description, the resistors will be consideredonly from the viewpoint of the necessary end supports for the throughbolts which carry the grids and these latter elements.

Referring now to Figs. 2 and 3 a resistor of the present inventionemploying the grids heretofore described, comprises essentially suitableend supports or frames 20 and 2l. One of these end frames such as 20 hasa pair of elongated supporting bolts 22 inserted through it and uponthese bolts are mounted insulating tubes 2l of mica or the like. Asuitable number of insulating washers or spacing collars 24 are thenplaced over the tube and in abutment with the inner face of wall 20 toestablish the spacing between said wall and the first grid lli, aterminal lug 25 which may constitute a mere perforated conductor platecontacting the ilrst grid and suitably arranged for connection with ater? minal 26 of a lead wire 21 being mounted in posi- -tion at thispoint. Thereafter, successive washers orl groups of washers or spacingcollars 24 of metal and of insulation are added to the stack vlugs areplaced at desired intervals to secure the desired different resistancesfor the Several taps and the entire stack is finally bound as a rigidunit in this case by drawing the remaining end frame 2| tightly upon thestack by means of nuts 28 on the threaded ends of the bolts. Thisparticular form of mounting may be used when the grids are stamped fromrelatively heavylgauge sheet metal, e. g., thicker than #18 gauge, andwith relatively wide spacing. A structure so arranged may, whenoverloaded to a degree that brings the grids to a white heat, show someslight warping of the grids while at that heat but this has been foundinsuilicient to cause the grids to touch and short circuit and it hasbeen found furthermore that upon cooling, the grids will return tosubstantially their original shape. It can be readily appreciated thatthe cost of resistors of this type is relatively low and thatcomparatively little skill is required in the assembly thereof.

It is preferred in general and not only for resistors employing verythin gauge grids, e. g., from #1.8 gauge to #26 gauge, to rigidly fixone of the bolts 22 in the end frames as specically illustrated in Fig.3 and to provide a floating mounting of the remaining bolt in the endframes as detailed in Fig. 6. This constructionis provided by forming anelongated slot 29 in each end frame in lieu of a perforation whichsnugly receives the bolt and providing means whereby the stack isrigidly clamped together with the bolt 22 and a means such as a collar3,0 and shoulder-forrning washers. which permits the rigidly clampedbolt and grid assembly to move longitudinally relative to the frame whenthe grids elongate slightly due to high heating. In ordinaryconstruction the heating of a grid to a white heat will increase thedistance between the rigidly anchored bolt and the floating bolt supportsuiiiciently to take care of expansion. Any other suitable mode ofproviding for the free floating of one of the support bolts may beemployed in lieu of the means just illustrated and described.

It will have been noted from the foregoing that the shape of the stampedgrids of electrical sheet metal has been designed to afford a relativelyuniform heating of the grids throughout the area thereof so that by theuse of floating bolt supports it is possible to simply and inexpensivelyconstruct resistors with only two supports for the bank in cases wherethe grid spaciny is threeeighths of an inch or more and the gauge of thegrids is of #18 or thicker. When it is desired to use closerspacings'and thinner grids than the foregoing, it is possible to use acenter bolt with spacer washers entered through one of the spaces I1between the loops oi' the grid. This would be provided essentially as amatter of safety against extreme and continued overheating due todifllculties in the electrical system or the load. The use of additionalsupports intermediate the ends of the grids and the necessary spacingwashers effects some restriction of the escape of heat although not tothe degree encountered in resistors now on the market and in which amultiplicity of supports are utilized.

As shown in Fig. 4 it is possible to assemble resistors employing doubleinterleaved banks of grids wherein the composite bank will have the sameresistance per unit of length as a single bank of similar grids at halfthe spacing of either the top or bottom section. Under some conditionssuch a double bank may be found ad` vantageous even though, by reason ofthe fact that the air temperature will reach its permitted maximumbefore the grids have reached their maximum permitted temperature, thecarrying capacity value is somewhat reduced. This is apparently due tothe restriction of air ilow caused by the use of additional centersupports 220. A s shown somewhat diagrammaticallyin Fig. 5 a pluralityof banks maybe arranged by suspending the first bank of grids fromarigidly mounted bolt 22 iixed between the elongated end frames 33, whichframes are provided with elongated lslots 34 at necessary intervals toreceive the remaining bolts 22. In such 'installation the intermediatebolts 2 2 would have adjacent sides of adjacent grid banks mountedthereon so that the entire assembly is suspended from the uppermostrigidly mounted support. One or more of these elongated banks may be ofcourse adaptable to assemblies which will t compactly into long narrowconfined spaces but nevertheless with very effective operation.

What is claimed is: v

l. As a new article of manufacture a resistance grid comprising anintegral sheet stamping having a pair of mounting lugs connected by anintermediate uniform strip forming a circuitous resistance path, saidstrip comprising a succession of parallel legged loops alternatelyextending in opposite directions from the longitudinal axis of the grid,the loopsv on each side of said axis being spaced apart by an openinggreater than the combined width of the parallel legs of the intermediateloop on the opposite side of said axis, each leg of each loop beingoffset laterally from every other leg of the remaining loops, and eachof said legs being longitudinally aligned with one of said openings.

2. A a new article of manufacture a resistance grid comprising` anintegral sheet stamping having a pair of mounting lugs connected by anintermediate uniform strip forming a circuitous resistance path, saidstrip comprising a succession of parallel sided loops alternatelyextending in opposite directions from the longitudinal axis of the grid.the loops on each side oi said axis being spaced apart by an openingshaped like the reversed counterpart of one of said loops, the interioropenings of the several loops on one side of the longitudinal center ofthe grid forming a continuation or the loop-spacing openings on theother side, the mounting lugs being disposed beyond the extreme loops ofsaid grid.

3. A resistance grid o! stamped sheet electrical resistance material andcomprising a pair of end mounting lugs connected by an intermediatesuccession of oppositelyextending loops, integral portions disposedangularly to the adjacent legs of successive loops and connecting saidadjacent legs whereby said legs are offset from one another in staggeredparallel relation and form, between alternate loops, enlargedventilating openings.

4. A resistor comprising spaced end frames having aligned perforationstherein at one side and aligned elongated slots in the other side, apair of suitable support bolts extending respectively. through the saidpair of perforations and the said pair of slots,-a suitably spaced stackof stamped sheet metal grids having perforate lugs at opposite endsreceiving the support bolts, said grids each having oppositely extendingopen loops in staggered relation on opposite sides of the longitudinalaxis of the grid and enlarged ventilating spaces on each side of eachloop, means rigidly securing together the perforate lugs on one end ofsaid stack, end frames and the supporting bolts in said perforations ofthe end frames and means rigidly securing the lugs on the remaining endof the stack on the remaining bolt. said bolt being adapted for movementin the slots in said end frames and permitting expansion of the grids.

5. As a new article of manufacture a resistance grid of stamped sheetmaterial comprising a pair of end lugs and a connected series of spacedloops disposed on opposite sides of the longitudinal axis oi the grid instaggered relation whereby enlarged openings are provided betweenadjacent loops on a common side of said axis, said enlarged openingscommunicating with smaller interior openings of intermediate loops onthe opposite side of said axis and providing, upon electricalenergization of the grid, increased heat dissipation resulting inuniform heating of all of said loops.

6. A resistor comprising spaced end frames each having aligned pairs ofelongated slots, support bolts each having opposite ends extending intoslots in opposed end frames. a suitably spaced stack of stamped sheetmetal grids having perforate lugs at opposite ends receiving the supportbolts, said grids having oppositely extending open loops in staggeredrelation on opposite sides of the longitudinal axis of the grid andenlarged Ventilating spaces on each side of each loop, means rigidlysecuring the grids to the bolts and means retaining the bolts againstaccldental separation from the end frames, said bolts being adapted formovement in the slots in said end frames and permitting expansion of thegrids.

MARVIN A. DU BOIS.

